Electroplating method

ABSTRACT

An electroplating method is capable of reliably embedding via holes with a plated metal such as copper or the like when a substrate with a seed layer of a metal having a greater ionization tendency than hydrogen is electroplated using an acidic plating solution such as a copper sulfate plating solution. The electroplating method including preparing a substrate having via holes covered with a first metal, which has a greater ionization tendency than hydrogen, in a surface thereof, pretreating the substrate by immersing the substrate in a pretreatment solution in which a second metal that is more noble than the first metal or a salt thereof is dissolved, and then electroplating the surface of the substrate to embed the second metal or a third metal in the via holes.

CROSS REFERENCE TO RELATED APPLICATIONS

This document claims priority to Japanese Application Number 2011-085883, filed Apr. 8, 2011 and Japanese Application Number 2012-043131, filed Feb. 29, 2012, the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electroplating method, and more particularly to an electroplating method which is useful for filling a metal, such as copper, into via holes in the manufacturing of a substrate, such as a semiconductor substrate or the like, which has a number of through-vias (via plug) vertically penetrating in its interior, and which can be used in so-called three-dimensional packaging of semiconductor chips.

2. Description of the Related Art

A technique of forming through-vias of a metal such as copper, vertically penetrating through a semiconductor substrate, is known as a method to electrically connect the layers of a multi-layer stack of semiconductor substrates.

FIGS. 1A through 1C show an exemplary process for producing a substrate having therein through-vias of copper. First, as shown in FIG. 1A, a substrate W is prepared by forming a plurality of upwardly-opening via holes 12 in a base 10, such as a silicon wafer, e.g., by using the lithography/etching technique, forming, e.g., by PVD, a dielectric film (not shown) on a surface of the base 10, including sidewalls of the via holes 12, forming a barrier layer 14 of a metal such as Ti (titanium) on the entire surface of the base 10, including interior surfaces of the via holes 12, and then forming a copper seed layer 16 on a surface of the barrier layer 14. The diameter “d” of the via holes 12 is, for example, 2 to 50 μm, in particular 10 to 20 μm, and the depth “h” of the via holes 12 is, for example, 20 to 150 μm.

Next, copper electroplating is carried out on the surface of the substrate W using the copper seed layer 16 as a cathode, thereby filling a plated metal (copper) 18 into the via holes 12 and depositing the plated metal 18 on the surface of the copper seed layer 16, as shown in FIG. 1B. In such a case where copper is embedded into the via holes 12 by copper electroplating, a copper sulfate plating solution, which is relatively inexpensive and is controlled relatively easy including its waste disposal, is widely used as a plating solution.

Thereafter, as shown in FIG. 1C, the extra plated metal 18, copper seed layer 16 and barrier layer 14 on the base 10 are removed, e.g., by chemical mechanical polishing (CMP). Further, the back surface side of the base 10 is polished away, e.g., to the position shown by the two-dot chain line in FIG. 1C, thereby exposing the bottom face of the plated metal 18 embedded in the via holes 12. The substrate W having therein through-vias of copper (plated metal 18), vertically penetrating through the substrate W, can be produced in this manner.

The via holes 12 generally have a high aspect ratio, i.e., the depth-to-diameter ratio, and have a large depth. In order to completely fill copper (plated metal) into such via holes 12, having a high aspect ratio and a large depth, by electroplating without producing defects such as voids in the embedded metal, it is usually necessary to perform the electroplating in a bottom-up manner of allowing the plated metal to grow preferentially from the bottoms of the via holes 12.

Such bottom-up plating is generally carried out by using a copper sulfate plating solution containing various additives such as SPS (bis(3-sulfopropyl)disulfide) as a plating accelerator, PEG (polyethylene glycol) as a suppressor, and PEI (polyethylene imine) as a leveler. These additives exert their effects after they are adsorbed onto a surface of a substrate.

PVD generally has a low step coverage. Therefore, if the continuous copper seed layer 16 is to be formed on the surface of the barrier layer 14 by PVD, then the continuous copper seed layer 16 needs to have a considerably large thickness in the range from 800 nm to 1000 nm, for example. Consequently, there has been a demand for seed layers of smaller thickness.

As shown in FIG. 2, it has been conceivable to form a cobalt film 20 as a seed layer, which doubles as a barrier layer, on an entire surface of a base 10 including the surfaces of via holes 12 by a conformal CVD process. Then, a copper electroplating process is carried out on the cobalt film 20 which acts as a cathode, embedding the via holes 12 with a plated metal, i.e., copper.

SUMMARY OF THE INVENTION

However, it is considerably difficult to completely fill the via holes 12, which have surfaces covered with the cobalt film 20 and have a high aspect ratio and a large depth, with a metal such as copper or like according to an electroplating process without producing unfilled regions.

For example, as shown in FIG. 3, a substrate W, whose surface is covered with a cobalt film and which has a plurality of via holes 12 a having a diameter of 10 μm and a depth of about 100 μm, is prepared. When a surface of this substrate W is electroplated using a copper sulfate plating solution containing various additives, embedding the via holes 12 a with a plated metal 18 of copper, unfilled regions 22 a with considerably rough surfaces tend to be formed in large bottom portions of the via holes 12 a.

FIG. 4 shows another substrate W whose surface is covered with a cobalt film and which has a plurality of via holes 12 b having a diameter of 20 μm and a depth of about 120 μm. When a surface of this substrate W is electroplated using a copper sulfate plating solution containing various additives, embedding the via holes 12 b with a plated metal 18 of copper, unfilled regions 22 b with considerably rough surfaces tend to be formed in large bottom portions of the via holes 12 b, and copper filled in the via holes 12 b are liable to have different lengths.

FIG. 5 shows still another substrate W whose surfaces is covered with a cobalt film and which has a plurality of via holes 12 c having a diameter of 30 μm and a depth of about 130 μm. When a surface of this substrate W is electroplated using a copper sulfate plating solution containing various additives, embedding the via holes 12 c with a plated metal 18 of copper, the plated metal 18 of copper is grown only on upper inner surfaces of the via holes 12 c. The grown plated metal 18 does not close the open ends of the via holes 12 c, failing to completely embed the via holes 12 c with the plated metal 18.

As described above, it is not easy to fully embed via holes, whose surfaces are covered with a cobalt film and which have a high aspect ratio and a large depth, with a plated metal of copper according to an electroplating process using a copper sulfate plating solution, without producing unfilled regions. It is believed that the reasons for the failure to fully embed the via holes are that during the electroplating process, the cobalt film is dissolved into the copper sulfate plating solution by direct contact with the copper sulfate plating solution which is acidic. Specifically, if the metal of a seed layer is a less noble metal such as cobalt, i.e., if the metal of a seed layer has a greater ionization tendency than hydrogen, then the seed layer is dissolved into an acidic plating solution.

The problem that the cobalt film, i.e., the seed layer, is dissolved into the plating solution manifests itself particularly on a contacting area of the outer circumferential surface of the substrate, which contacts a sealing member that protects or seals the outer circumferential surface from the plating solution, and its neighborhood. Specifically, the cobalt film is dissolved at a higher rate at the contacting area of the outer circumferential surface of the substrate and its neighborhood, because a plated film is less likely to be deposited on those localized areas due to the electric field being blocked in those localized areas. The substrate is generally supplied with an electric current from contacts that come into contact with the outer circumferential surface of the substrate which is protected from the plating solution by the sealing member. If the cobalt film on the contacting area of the outer circumferential surface of the substrate and its neighborhood is dissolved away, then the electric current is prevented from flowing to the substrate in the absence of the seed layer, i.e., the cobalt film.

One solution would be to use a copper pyrophosphate plating solution which is alkaline. However, the copper pyrophosphate plating solution is more expensive than the copper sulfate plating solution, and makes the plating solution management including waste disposal complex.

The problem that the seed layer is dissolved into the acidic plating solution arises not only when a surface of a cobalt film is plated with copper, but also when a surface of a metal having a greater ionization tendency than hydrogen is plated with another metal.

The present invention has been made in view of the above situation. It is therefore an object of the present invention to provide an electroplating method which is capable of reliably embedding via holes with a plated metal such as copper or the like when a substrate with a seed layer of a metal having a greater ionization tendency than hydrogen is electroplated using an acidic plating solution such as a copper sulfate plating solution.

In order to achieve the above object, the present invention provides an electroplating method comprising preparing a substrate having via holes covered with a first metal, which has a greater ionization tendency than hydrogen, in a surface thereof, pretreating the substrate by immersing the substrate in a pretreatment solution in which a second metal that is more noble than the first metal or a salt thereof is dissolved, and then electroplating a surface of the substrate to embed the second metal or a third metal in the via holes.

Since the substrate, which has via holes covered with a first metal that has a greater ionization tendency than hydrogen in the surface, is pretreated by immersing the substrate in a pretreatment solution in which a second metal that is more noble than the first metal or a salt thereof is dissolved, a metal film of the second metal that is more noble than the first metal is formed on the surface of the first metal by way of replacement. When the surface of the substrate is electroplated, the first metal is prevented from directly contacting an acidic plating solution and being dissolved into the acidic plating solution, thereby reliably embedding the plated metal in the via holes.

The pretreatment solution should preferably be in the range from a mildly acidic level to a mildly alkaline level thereby to prevent the first film, such as a cobalt film, from being dissolved into the pretreatment solution upon contact therewith.

Preferably, the substrate should be pretreated by immersing the substrate in deaerated water before pretreating the substrate by immersing the substrate in the pretreatment solution in which the second metal or the salt thereof is dissolved.

This can enhance a hydrophilicity of the surface of the substrate, and after penetrative deaerated water has entered the via holes, the penetrative deaerated water in the via holes can be replaced with the pretreatment solution, which thus reliably enters the via holes.

The first metal may be cobalt, for example, and the second metal which is more noble than cobalt may be copper. In this case, the pretreatment solution should preferably have a copper concentration ranging from 1 to 70 g/L.

With a metal film of copper being formed by way of replacement on the surface of the cobalt film, even if an acidic copper sulfate solution is used as a plating solution, the cobalt film is prevented from being dissolved into the plating solution. The pretreatment solution containing copper may be prepared by dissolving copper sulfate into pure water, for example, and the copper concentration in the pretreatment solution is set to a desired value depending on the size of the via holes.

The second metal may be palladium, cold, platinum, or silver, and the third metal may be copper. In this case, an acidic copper sulfate plating solution is preferably used as the plating solution.

According to the present invention, when the first metal, e.g., cobalt, which has a greater ionization tendency than hydrogen is used as a seed layer, and the substrate is electroplated using an acidic plating solution such as a copper sulfate plating solution, the first metal is protected from the plating solution by the metal film of the second metal which is more noble than the first metal and which is formed on the surface of the first metal by way of replacement. The via holes covered with the first metal are thus reliably embedded with the plated metal of copper or the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A through 1C are cross-sectional views showing a process of fabricating a substrate having a plurality of through-vias of copper which extend vertically through the substrate;

FIG. 2 is a cross-sectional view showing a substrate whose surface is covered with a cobalt film and which has via holes;

FIG. 3 is a cross-sectional view showing a substrate, whose surface is covered with a cobalt film and which has a plurality of via holes having a diameter of 10 μm and a depth of about 100 μm, at the time the substrate is electroplated using a copper sulfate plating solution, embedding the via holes with a plated metal of copper;

FIG. 4 is a cross-sectional view showing a substrate, whose surface is covered with a cobalt film and which has a plurality of via holes having a diameter of 20 μm and a depth of about 120 μm, at the time the substrate is electroplated using a copper sulfate plating solution, embedding the via holes with a plated metal of copper;

FIG. 5 is a cross-sectional view showing a substrate, whose surface is covered with a cobalt film and which has a plurality of via holes having a diameter of 30 μm and a depth of about 130 μm, at the time the substrate is electroplated using a copper sulfate plating solution, embedding the via holes with a plated metal of copper;

FIG. 6 is a schematic plan view showing the overall arrangement of a plating facility used for carrying out an electroplating method of the present invention;

FIG. 7 is a schematic elevational view of a transfer robot employed in the plating facility shown in FIG. 6;

FIG. 8 is a schematic cross-sectional view of a plating device employed in the plating facility shown in FIG. 6;

FIG. 9 is a plan view of a stirring paddle (stirring tool) of the plating device shown in FIG. 8;

FIG. 10 is a cross-sectional view taken alone line A-A of FIG. 9;

FIG. 11 is a cross-sectional view showing a substrate, whose surface is covered with a cobalt film and which has a plurality of via holes having a diameter of 10 μm and a depth of about 100 μm, at the time the substrate is electroplated using a copper sulfate plating solution, embedding the via holes with a plated metal of copper, after the substrate has been pretreated using a pretreatment solution with a copper concentration of 10 g/L;

FIG. 12 is a cross-sectional view showing a substrate, whose surface is covered with a cobalt film and which has a plurality of via holes having a diameter of 20 μm and a depth of about 120 μm, at the time the substrate is electroplated using a copper sulfate plating solution, embedding the via holes with a plated metal of copper, after the substrate has been pretreated using a pretreatment solution with a copper concentration of 10 g/L;

FIG. 13 is a cross-sectional view showing a substrate, whose surface is covered with a cobalt film and which has a plurality of via holes having a diameter of 30 μm and a depth of about 130 μm, at the time the substrate is electroplated using a copper sulfate plating solution, embedding the via holes with a plated metal of copper, after the substrate has been pretreated using a pretreatment solution with a copper concentration of 10 g/L;

FIG. 14 is a cross-sectional view showing a substrate, whose surface is covered with a cobalt film and which has a plurality of via holes having a diameter of 10 μm and a depth of about 100 μm, at the time the substrate is electroplated using a copper sulfate plating solution, embedding the via holes with a plated metal of copper, after the substrate has been pretreated using a pretreatment solution with a copper concentration of 30 g/L;

FIG. 15 is a cross-sectional view showing a substrate, whose surface is covered with a cobalt film and which has a plurality of via holes having a diameter of 20 μm and a depth of about 120 μm, at the time the substrate is electroplated using a copper sulfate plating solution, embedding the via holes with a plated metal of copper, after the substrate has been pretreated using a pretreatment solution with a copper concentration of 30 g/L;

FIG. 16 is a cross-sectional view showing a substrate, whose surface is covered with a cobalt film and which has a plurality of via holes having a diameter of 10 μm and a depth of about 100 μm, at the time the substrate is electroplated using a copper sulfate plating solution, embedding the via holes with a plated metal of copper, after the substrate has been pretreated using a pretreatment solution with a copper concentration of 60 g/L;

FIG. 17 is a cross-sectional view showing a substrate, whose surface is covered with a cobalt film and which has a plurality of via holes having a diameter of 20 μm and a depth of about 120 μm, at the time the substrate is electroplated using a copper sulfate plating solution, embedding the via holes with a plated metal of copper, after the substrate has been pretreated using a pretreatment solution with a copper concentration of 60 g/L; and

FIG. 18 is a cross-sectional view showing a substrate, whose surface is covered with a cobalt film and which has a plurality of via holes having a diameter of 30 μm and a depth of about 130 μm, at the time the substrate is electroplated using a copper sulfate plating solution, embedding the via holes with a plated metal of copper, after the substrate has been pretreated using a pretreatment solution with a copper concentration of 60 g/L.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will now be described with reference to the drawings. The following description illustrates an exemplary case in which a substrate W, as shown in FIG. 2, with a cobalt film 20 formed on the entire surface thereof as a seed layer, which doubles as a barrier layer, including surfaces of via holes 12 by a conformal CVD process, is prepared. Then, a copper electroplating process is carried out on a surface of the substrate W using a copper sulfate plating solution, thus embedding the via holes 12 with a plated metal, i.e., copper, thereby to form through-vias of copper in the substrate W.

FIG. 6 is an overall layout plan view of a plating facility used for carrying out an electroplating method of the present invention. This plating facility is designed so as to automatically perform all plating processes including pretreatment of a substrate, plating, and post-treatment of the plating, in a successive manner. The interior of an apparatus frame 110 having an armored panel attached thereto is divided by a partition plate 112 into a plating space 116 for performing a plating process of a substrate and treatments of the substrate to which a plating solution is attached, and a clean space 114 for performing other processes, i.e., processes not directly involving a plating solution. Two substrate holders 160 (see FIG. 7) are arranged in parallel, and substrate attachment/detachment stages 162 to attach a substrate to and detach a substrate from each substrate holder 160 are provided as a substrate delivery section on a partition portion partitioned by the partition plate 112, which divides the plating space 116 from the clean space 114. Loading/unloading ports 120, on which substrate cassettes storing substrates are mounted, are connected to the clean space 114. Further, the apparatus frame 110 has a console panel 121 provided thereon.

In the clean space 114, there are disposed an aligner 122 for aligning an orientation flat or a notch of a substrate with a predetermined direction, two cleaning/drying devices 124 for cleaning a plated substrate and rotating the substrate at a high speed to spin-dry the substrate. Further, a first transfer robot 128 is disposed substantially at the center of these processing devices, i.e. the aligner 122 and the cleaning/drying devices 124, to thereby transfer and deliver a substrate between the processing devices 122, 124, the substrate attachment/detachment stages 162, and the substrate cassettes mounted on the loading/unloading ports 120.

The aligner 122 and the cleaning/drying devices 124 disposed in the clean space 114 are designed so as to hold and process a substrate in a horizontal state in which a front face of the substrate faces upward. The first transfer robot 128 is designed so as to transfer and deliver a substrate in a horizontal state in which a front face of the substrate faces upward.

In the plating space 116, in the order from the partition plate 112, there are disposed a stocker 164 for storing or temporarily storing the substrate holders 160, a first pretreatment device 126 for carrying out a pre-wetting treatment (first pretreatment) for enhancing a hydrophilicity of the surface of the substrate by wetting with the deaerated water of e.g., pure water (DIW), a second pretreatment device 166 for carrying out a replacing treatment (second pretreatment) for replacing cobalt on a surface of the cobalt film with copper by immersing the substrate in a mildly acidic or mildly alkaline pretreatment solution containing copper which is more noble than cobalt, a first water-cleaning device 168 a for cleaning the surface of the substrate with pure water, a electroplating device 170 for carrying out electroplating, a second water-cleaning device 168 b, and a blowing device 172 for dewatering the plated substrate. Two second transfer robots 174 a, 174 b are disposed beside these devices so as to be movable along a rail 176. One of the second transfer robots 174 a transfers the substrate holders 160 between the substrate attachment/detachment stages 162 and the stocker 164. The other of the second transfer robots 174 b transfers the substrate holders 160 between the stocker 164, the first pretreatment device 126, the second pretreatment device 166, the first water-cleaning device 168 a, the electroplating device 170, the second water-cleaning device 168 b, and the blowing device 172.

As shown in FIG. 7, each of the second transfer robots 174 a, 174 b has a body 178 extending in a vertical direction and an arm 180 which is vertically movable along the body 178 and rotatable about its axis. The arm 180 has two substrate holder retaining portions 182 provided in parallel for detachably retaining the substrate holders 160. The substrate holder 160 is designed so as to hold a substrate W in a state in which a front face of the substrate is exposed while a peripheral portion of the substrate is sealed, and to be capable of attaching the substrate W to the substrate holder 160 and detaching the substrate W from the substrate holder 160.

The stocker 164, the first pretreatment device 126, the second pretreatment device 166, the water-cleaning devices 168 a, 168 b, and the electroplating device 170 are designed so as to engage with outwardly projecting portions 160 a provided at both ends of each substrate holder 160 to thus support the substrate holders 160 in such a state that the substrate holders 160 are suspended in a vertical direction.

The first pretreatment device 126 has two pretreatment tanks 127 for holding therein a deaerated water, such as pure water (deaerated DIW) having, e.g., a dissolved oxygen concentration of not more than 2 mg/L or the like. As shown in FIG. 7, the arm 180 of the second transfer robot 174 b holding the substrate holders 160, which are loaded with the substrates W in a vertical state, is lowered so as to engage with upper ends of the first pretreatment tanks 127 to support the substrate holders 160 in a suspended manner. Thus, the first pretreatment device 126 is designed so that the substrate holders 160 are immersed together with the substrates W in the deaerated water in the pretreatment tanks 127 to carry out a pre-wetting treatment (first pretreatment) of the surface of the substrate.

The second pretreatment device 166 has two pretreatment tanks 183 for holding therein a mildly acidic or mildly alkaline pretreatment solution containing copper which is more noble than cobalt. As shown in FIG. 7, the arm 180 of the second transfer robot 174 b holding the substrate holders 160, which are loaded with the substrates W in a vertical state, is lowered so as to engage with upper ends of the pretreatment tanks 183 to support the substrate holders 160 in a suspended manner. Thus, the second pretreatment device 166 is designed so that the substrate holders 160 are immersed together with the substrates W in the pretreatment liquid in the pretreatment tanks 183 to carry out a replacing treatment (second pretreatment) of a surface of a cobalt film.

A pretreatment solution, in which copper is dissolved and which is in the range from a mildly acidic level to a mildly alkaline level, i.e., has a pH in the range from 2 to 9, is prepared by dissolving copper sulfate into pure water, for example. A copper concentration of the pretreatment solution may be in a range from 1 to 70 g/L, and is set to a desired value depending on the size of the via holes or the like. A buffering agent for pH adjustment, such as phosphate, phthalate, citrate, succinate or boracic acid, may be added to the pretreatment solution to suppress pH fluctuation of the pretreatment solution.

Similarly, the water-cleaning devices 168 a, 168 b have two water-cleaning tanks 184 a and two water-cleaning tanks 184 b which hold pure water therein, respectively, and the electroplating device 170 has a plurality of plating tanks 186 which hold a plating solution therein. The water-cleaning devices 168 a, 168 b and the electroplating device 170 are designed so that the substrate holders 160 are immersed together with the substrates W in the pure water in the water-cleaning tanks 184 a, 184 b or the plating solution in the plating tanks 186 to carry out water-cleaning or plating in the same manner as described above. The arm 180 of the second transfer robot 174 b holding the substrate holders 160, which are loaded with substrates W in a vertical state, is lowered, and air or inert gas is injected toward the substrates W mounted on the substrate holders 160 to blow away a liquid attached to the substrate holders 160 and the substrates W and to dewater the substrates W. Thus, the blowing device 172 is designed so as to carry out blowing treatment.

As shown in FIG. 8, each plating tank 186 provided in the electroplating device 170 is designed so as to hold a predetermined amount of plating solution Q therein. The substrates W, which are held in a state such that the front faces (surfaces to be plated) are exposed while peripheral portions of the substrates are watertightly sealed by the substrate holder 160, are immersed in the plating solution Q in a vertical direction. In this embodiment, an acidic copper sulfate plating solution which, in addition to copper ions, a supporting electrolyte and halogen ions, contains various additives such as SPS (bis(3-sulfopropyl)disulfide) as a plating accelerator, PEG (polyethylene glycol) as a suppressor, and PEI (polyethylene imine) as a leveler, for example, is used as the plating solution Q. Sulfuric acid is preferably used as the supporting electrolyte, and chlorine ions are preferably used as the halogen ions.

An overflow tank 200 for receiving the plating solution Q that has overflowed an edge of the plating tank 186 is provided around an upper end of the plating tank 186. One end of a circulation piping 204, which is provided with a pump 202, is connected to a bottom of the overflow tank 200, and the other end of the circulation piping 204 is connected to a plating solution supply inlet 186 a provided at a bottom of the plating tank 186. Thus, the plating solution Q in the overflow tank 200 is returned into the plating tank 186 by the actuation of the pump 202. Located downstream of the pump 202, a constant-temperature unit 206 for controlling the temperature of the plating solution Q and a filter 208 for filtering out foreign matter contained in the plating solution are interposed in the circulation piping 204.

A bottom plate 210, having a large number of plating solution passage holes therein, is installed in the bottom of the plating tank 186. The interior of the plating tank 186 is thus separated by the bottom plate 210 into an upper substrate processing chamber 214 and a lower plating solution distribution chamber 212. Further, a shield plate 216, extending vertically downward, is mounted to the lower surface of the bottom plate 210.

According to this electroplating device 170, the plating solution Q is introduced into the plating solution distribution chamber 212 of the plating tank 186 by the actuation of the pump 202, flows into the substrate processing chamber 214 passing through the plating solution passage holes provided in the bottom plate 210, flows vertically approximately parallel to the surface of the substrate W held by the substrate holder 160, and then flows into the overflow tank 200.

An anode 220 having a circular shape corresponding to the shape of the substrate W is held by an anode holder 222 and provided vertically in the plating tank 186. When the plating solution Q is filled in the plating tank 186, the anode 220 held by the anode holder 222 becomes immersed in the plating solution Q in the plating tank 186 and faces the substrate W held by the substrate holder 160 and disposed in the plating tank 186.

Further, in the plating tank 186, a regulation plate 224, for regulating the distribution of electric potential in the plating tank 186, is disposed between the anode 220 and the substrate W to be disposed at a predetermined position in the plating tank 186. In this embodiment, the regulation plate 224 is comprised of a cylindrical portion 226 and a rectangular flange portion 228, and is made of polyvinyl chloride that is a dielectric material. The cylindrical portion 226 has such an opening size and axial length as to sufficiently restrict broadening of electric field. A lower end of the flange portion 228 of the regulation plate 224 reaches the bottom plate 210.

Further, in the plating tank 186, a regulation plate 224, for regulating the distribution of electric potential in the plating tank 186, is disposed between the anode 220 and the substrate W to be disposed at a predetermined position in the plating tank 186. In this embodiment, the regulation plate 224 is comprised of a cylindrical portion 226 and a rectangular flange portion 228, and is made of polyvinyl chloride that is a dielectric material. The cylindrical portion 226 has such an opening size and axial length as to sufficiently restrict broadening of electric field. A lower end of the flange portion 228 of the regulation plate 224 reaches the bottom plate 210.

Between the regulating plate 224 and the substrate W to be disposed at a predetermined position in the plating tank 186 is disposed a vertically-extending stirring paddle 232 as a stirring tool which reciprocates parallel to the surface of the substrate W to stir the plating solution Q between the substrate W and the regulating plate 224. By stirring the plating solution Q with the stirring paddle (stirring tool) 232 during plating, a sufficient amount of copper ions can be supplied uniformly to the surface of the substrate W.

As shown in FIGS. 9 and 10, the stirring paddle 232 is comprised of a rectangular plate-like member having a uniform thickness “t” of 3 to 5 mm, and has a plurality of parallel slits 232 a that define vertically-extending strip-like portions 232 b. The stirring paddle 232 is formed of, for example, a resin such as PVC, PP and PTFE, and SUS or titanium with a Teflon coating. It is preferred that at least part of the stirring paddle 232, which contacts the plating solution, be electrically isolated. The vertical length L₁ of the stirring paddle 232 and the vertical length L₂ of the slits 232 a are sufficiently larger than the vertical size of the substrate W. Further, the stirring paddle 232 is so designed that the sum of its lateral length H and its reciprocation distance (stroke) is sufficiently larger than the lateral size of the substrate W.

It is preferred that the width and the number of the slits 232 a be determined such that each strip-shaped portion 232 b is as narrow as possible insofar as it has the necessary rigidity so that the strip-shaped portions 232 b between the slits 232 a can efficiently stir the plating solution and, in addition, the plating solution can efficiently pass through the slits 232 a.

The electroplating device 170 is provided with a plating power source 250 of which the positive pole is connected via a conducting wire to the anode 220 and the negative pole is connected via a conducting wire to the surface of the substrate W during plating. The plating power source 250 is connected to a control section 252, and the electroplating device 170 is controlled based on signals from the control section 252.

A series of plating processes, to be carried out by the plating facility shown in FIG. 6, for electroplating the surface of the substrate W with the cobalt film 20 formed on the entire surface thereof including the surfaces of the via holes 12 as shown in FIG. 2, using a copper sulfate plating solution, thus embedding the via holes 12 with a plated metal, i.e., copper, will be described below.

First, the substrate W is placed, with its front surface (surface to be plated) facing upwardly, in a substrate cassette, and the substrate cassette is mounted on the loading/unloading port 120. One of the substrates W is taken out of the substrate cassette mounted on the loading/unloading port 120 by the first transfer robot 128 and placed on the aligner 122 to align an orientation flat or a notch of the substrate W with a predetermined direction. On the other hand, two substrate holders 160, which have been stored in a vertical state in the stocker 164, are taken out by the second transfer robot 174 a, rotated through 90° so that the substrate holders 160 are brought into a horizontal state, and then placed in parallel on the substrate attachment/detachment stages 162.

The substrates W aligned the orientation flat or the notch thereof with a predetermined direction are transferred and loaded into the substrate holders 160 placed on the substrate attachment/detachment stages 162 in a state such that peripheral portions of the substrates are sealed. The two substrate holders 160, which have been loaded with the substrates W, are simultaneously retained, lifted, and then transferred to the stocker 164 by the second transfer robot 174 a. The substrate holders 160 are rotated through 90° into a vertical state and lowered so that the two substrate holders 160 are held (temporarily stored) in the stocker 164 in a suspended manner. The above operation is carried out repeatedly in a sequential manner, so that substrates are sequentially loaded into the substrate holders 160, which are stored in the stocker 164, and are sequentially held (temporarily stored) in the stocker 164 at predetermined positions in a suspended manner.

On the other hand, the two substrate holders 160, which have been loaded with the substrates and temporarily stored in the stocker 164, are simultaneously retained, lifted, and then transferred to the first pretreatment device 126 by the second transfer robot 174 b. In the first pretreatment device 126, the substrates W held by the substrate holders 160 are immersed in deaerated water such as pure water (DIW) in the pretreatment tanks 127 to thereby carry out a pre-wetting treatment (first pretreatment) on the surface of the substrate W. Pure water used as the deaerated water has its dissolved oxygen concentration adjusted preferably to 2 mg/L or lower by a vacuum deaerator. By thus carrying out the pre-wetting treatment (first treatment) on the surface of the substrate W, a hydrophilicity of the surface of the substrate W can be enhanced, allowing highly penetrative deaerated water to enter the via holes 12 in the substrates W.

After the first pretreatment, the two substrate holders 160, each loaded with the substrate, are transferred to the second pretreatment device 166 in the same manner as described above, where the substrates are immersed in a pretreatment solution, which contains copper dissolved therein and is in ranging from a mildly acidic level to a mildly alkaline level, in the pretreatment tanks 183 to thereby carry out a replacing treatment (second pretreatment) on the surface of the cobalt film 20. In this replacing treatment (second pretreatment), the deaerated water, which has entered the via holes 12 in the substrates W by performing the pre-wetting treatment (first pretreatment), is replaced with the pretreatment solution, and the via holes 12 are securely filled with the pretreatment solution.

The replacing treatment will be described in detail below. When each of the substrates W whose surface is covered with the cobalt film 20 is immersed in the pretreatment solution containing copper, cobalt, which is less noble than copper, on the surface of the cobalt film 20 is replaced with copper in the pretreatment solution, thereby covering the surface of the cobalt film 20 with copper. The surface of the cobalt film 20, which is covered with copper, will be protected from a plating solution. When copper covers the entire surface of the cobalt film 20, the replacing reaction stops, and a very thin copper film is deposited on the surface of the cobalt film 20. The thickness of the cobalt film 20 is essentially not affected by the copper film, so that the cobalt film 20 has its function as a barrier layer remaining effective or unimpaired.

The pretreatment solution should preferably be in the range from a mildly acidic level to a mildly alkaline level to prevent the cobalt film 20 from being dissolved into the pretreatment solution upon contact therewith. The pretreatment solution may have its dissolved oxygen concentration adjusted to 2 mg/L or lower, as with the deaerated water.

After the second pretreatment, the substrate holders 160, each loaded with the substrate W, are transferred to the first water-cleaning device 168 a in the same manner as described above, where the surfaces of the substrates W are cleaned with pure water held in the first water-cleaning tanks 184 a.

After the water cleaning, the two substrate holders 160, each loaded with the substrate W, are transferred to above the plating tanks 186 of the electroplating device 170 in the same manner as described above. The plating tanks 186 have been filled with a predetermined amount of plating solution Q having a predetermined composition, the plating solution being circulated through the circulation system. The substrate holders 160 are then lowered to immerse the substrates W, held by the substrate holders 160, in the plating solution Q in the plating tanks 186. Each substrate W is disposed in the plating solution Q at a position facing the anode 220 held by the anode holder 222.

Each substrates W is immersed in the plating solution Q for a predetermined period of time, replacing the pretreatment solution in the via holes 12 with the plating solution W. During the predetermined period of time, no voltage is applied between the anode 220 and the cobalt film 20, covered with the copper film, on the surface of each substrate W. Thereafter, a voltage is applied between the anode 220 and the cobalt film 20, covered with the copper film, on the surface of each substrate W, performing an electroplating process to deposit a plated film of copper on the surface of the cobalt film 20 covered with the copper film, embedding the via holes 12 with the plated film of copper.

The plating solution Q used in the electroplating process is an acidic copper sulfate plating solution whose management is relatively easy to carry out. If the cobalt film 20 is brought into direct contact with the acidic copper sulfate plating solution, then the cobalt film 20 is dissolved into the plating solution. In this embodiment, however, since the surface of the cobalt film 20 is covered with a very thin film of copper, the cobalt film 20 is kept out of direct contact with the acidic copper sulfate plating solution, and hence is prevented from being dissolved into the plating solution Q. After the voltage has been applied between the anodes 220 and the cobalt films 20 on the surfaces of the substrates W for a predetermined period time, the electroplating process is terminated.

After the substrates W have been immersed in the plating solution Q and until the electroplating process is terminated, the stirring paddles 232 are moved back and forth parallel to the substrates W to stir the plating solution Q between the regulation plates 224 and the substrates W. The embedding of the via holes 12 goes on until the aspect ratio of the via holes 12 is reduced to the extent that the plating solution Q can easily reach the surface of the plated metal in the via holes 12. If the plating solution Q is still strongly stirred by the stirring paddles 232 at this time, then the growth of the plated metal may be slowed down, requiring an extra time until the via holes 12 are fully embedded. To avoid the shortcoming, it is desirable that the plating solution Q be stirred less intensively when the plating process has progressed to a certain extent.

After the electroplating process is terminated, the voltage applied between the anode 220 and the cobalt film 20, covered with the copper film, on the surface of each substrate W, is switched off. Thereafter, the two substrate holders 160, each loaded with the substrate W, are held again by the second transfer robot 174 b and withdrawn from the plating tanks 186.

The two substrate holders 160 are then transferred to the second water-cleaning device 168 b in the same manner as described above, where the surfaces of the substrates are cleaned by immersing the substrates in pure water held in the water-cleaning tanks 184 b. Thereafter, the substrate holders 160, each loaded with the substrate, are transferred to the blowing device 172 in the same manner as described above, where the plating solution and water droplets are removed from the substrate holders 160 by blowing air or an inert gas onto the substrate holders 160. Thereafter, the substrate holders 160, each loaded with the substrate, are returned to the stocker 164 and are each suspended and held at a predetermined position in the stocker 164 in the same manner as described above.

The second transfer robot 174 b sequentially repeats the above operations to sequentially return substrate holders 160, each loaded with a substrate after electroplating, to predetermined positions in the stocker 164 and suspend the substrate holders 160 in the stocker 164. On the other hand, two substrate holders 160 loaded with substrates after electroplating, which have been returned to the stocker 164, are simultaneously gripped by the second transfer robot 174 a, and are placed on the substrate attachment/detachment stages 162 in the same manner as described above.

The first transfer robot 128, disposed in the clean space 114, takes a substrate out of a substrate holder 160 placed on one of the substrate attachment/detachment stages 162 and transfers the substrate to one of the cleaning/drying devices 124. In the cleaning/drying device 124, the substrate, which is held in a horizontal position with the front surface facing upwardly, is cleaned, e.g., with pure water and then spin-dried by rotating it at a high speed. Thereafter, the substrate is returned by the first transfer robot 128 to the substrate cassette mounted on the loading/unloading port 120, thereby completing the sequence of electroplating operations.

An experiment was conducted in which a plurality of types of substrates whose entire surfaces, including the surfaces of via holes of different sizes, are covered with a cobalt film were prepared. The pre-wetting treatment (first pretreatment) was carried out to immerse the substrates in deaerated water, followed by the replacing treatment (second pretreatment) for replacing the cobalt film using a plurality of types of pretreatment solutions having different copper concentrations. Thereafter, the substrates were electroplated to embed the via holes with a plated metal of copper. The embedding of the via holes in the experiment will be described below.

FIG. 11 shows a substrate W, whose surface is covered with a cobalt film and which has a plurality of via holes 12 a having a diameter of 10 μm and a depth of about 100 μm, at the time the substrate W is electroplated using a copper sulfate plating solution, embedding the via holes 12 a with a plated metal 18 of copper, after a pre-wetting treatment (first pretreatment) was carried out to immerse the substrate W in deaerated water for 10 minutes and then a replacing treatment (second pretreatment) was carried out to replace the surface of the cobalt film for 1 minute using a pretreatment solution with a copper concentration of 10 g/L. FIG. 12 shows a substrate W, whose surface is covered with a cobalt film and which has a plurality of via holes 12 b having a diameter of 20 μm and a depth of about 120 μm, at the time the substrate W is electroplated using a copper sulfate plating solution, embedding the via holes 12 b with a plated metal 18 of copper, after the pre-wetting treatment and the replacing treatment were carried out in the same manner as with FIG. 12. FIG. 13 shows a substrate W, whose surface is covered with a cobalt film and which has a plurality of via holes 12 c having a diameter of 30 μm and a depth of about 130 μm, at the time the substrate W is electroplated using a copper sulfate plating solution, embedding the via holes 12 c with a plated metal 18 of copper, after the pre-wetting treatment and the replacing treatment were carried out in the same manner as with FIG. 12.

It can be seen from FIGS. 11 through 13 that when the replacing treatment (second pretreatment) was carried out to replace the surface of the cobalt film for 1 minute using a pretreatment solution with a copper concentration of 10 g/L, the via holes 12 a having a diameter of 10 μm and a depth of 100 μm, as shown in FIG. 11, were embedded to different depths, the via holes 12 c having a diameter of 30 μm and a depth of 130 μm, as shown in FIG. 13, were embedded producing unfilled regions deeply in the via holes 12 c, and the via holes 12 b having a diameter of 20 μm and a depth of 120 μm, as shown in FIG. 12, were well embedded.

The pretreatment solution with a copper concentration of 10 g/L is prepared by dissolving copper sulfate into pure water. A pretreatment solution with a copper concentration of 30 g/L and a pretreatment solution with a copper concentration of 60 g/L, to be described below, is also prepared by dissolving copper sulfate into pure water.

FIG. 14 shows a substrate W, whose surface is covered with a cobalt film and which has a plurality of via holes 12 a having a diameter of 10 μm and a depth of about 100 μm, at the time the substrate W is electroplated using a copper sulfate plating solution, embedding the via holes 12 a with a plated metal 18 of copper, after a pre-wetting treatment (first pretreatment) was carried out to immerse the substrate W in deaerated water for 10 minutes and then a replacing treatment (second pretreatment) was carried out to replace the surface of the cobalt film for 1 minute using a pretreatment solution with a copper concentration of 30 g/L. FIG. 15 shows a substrate W, whose surface is covered with a cobalt film and which has a plurality of via holes 12 b having a diameter of 20 μm and a depth of about 120 μm, at the time the substrate W is electroplated using a copper sulfate plating solution, embedding the via holes 12 b with a plated metal 18 of copper, after the pre-wetting treatment and the replacing treatment were carried out in the same manner as with FIG. 14.

It can be seen from FIGS. 14 and 15 that when the replacing treatment (second pretreatment) was carried out to replace the surface of the cobalt film for 1 minute using a pretreatment solution with a copper concentration of 30 g/L, the via holes 12 a having a diameter of 10 μm and a depth of 100 μm, as shown in FIG. 14, were embedded uniformly to considerable depths while producing small unfilled regions deeply in the via holes 12 a, and via holes 12 b having a diameter of 20 μm and a depth of 120 μm, as shown in FIG. 15, were substantially fully embedded.

FIG. 16 shows a substrate W, whose surface is covered with a cobalt film and which has a plurality of via holes 12 a having a diameter of 10 μm and a depth of about 100 μm, at the time the substrate W is electroplated using a copper sulfate plating solution, embedding the via holes 12 a with a plated metal 18 of copper, after a pre-wetting treatment (first pretreatment) was carried out to immerse the substrate W in deaerated water for 10 minutes and then a replacing treatment (second pretreatment) was carried out to replace the surface of the cobalt film for 1 minute using a pretreatment solution with a copper concentration of 60 g/L. FIG. 17 shows a substrate W, whose surface is covered with a cobalt film and which has a plurality of via holes 12 b having a diameter of 20 μm and a depth of about 120 μm, at the time the substrate W is electroplated using a copper sulfate plating solution, embedding the via holes 12 b with a plated metal 18 of copper, after the pre-wetting treatment and the replacing treatment were carried out in the same manner as with FIG. 16. FIG. 18 shows a substrate W, whose surface is covered with a cobalt film and which has a plurality of via holes 12 c having a diameter of 30 μm and a depth of about 130 μm, at the time the substrate W is electroplated using a copper sulfate plating solution, embedding the via holes 12 c with a plated metal 18 of copper, after the pre-wetting treatment and the replacing treatment were carried out in the same manner as with FIG. 16.

It can be seen from FIGS. 16 through 18 that when the replacing process (second pretreatment) was carried out to replace the surface of the cobalt film for 1 minute using a pretreatment solution with a copper concentration of 60 g/L, the via holes 12 a having a diameter of 10 μm and a depth of 100 μm, as shown in FIG. 16, were embedded producing small unfilled regions deeply in the via holes 12 a, the via holes 12 b having a diameter of 20 μm and a depth of 120 μm, as shown in FIG. 17, and the via holes 12 c having a diameter of 30 μm and a depth of 130 μm, as shown in FIG. 18, were well embedded.

In the above embodiment, cobalt is used as a first metal having a greater ionization tendency than hydrogen, and copper is used as a second metal which is more noble than cobalt in embedding via holes with copper (second metal) according to a plating process. A pretreatment solution containing copper is prepared by dissolving copper sulfate contained in a plating solution into pure water. Since the pretreatment solution is prepared using a component contained in the plating solution, it is not necessary to take into account the carrying of unwanted materials into subsequent processes. Palladium, gold, platinum, or silver may be used as a metal (second metal) that is more noble than cobalt (first metal), and via holes may be embedded with copper (third metal) according to a plating process.

In the above embodiment, the pre-wetting treatment (first pretreatment) for immersing a substrate in deaerated water such as pure water (DIW) to enhance a hydrophilicity of a surface of the substrate and the replacing treatment (second pretreatment) for immersing a substrate in a mildly acidic or mildly alkaline pretreatment solution containing copper, which is more noble than cobalt, to replace cobalt in a surface of a cobalt film with copper are carried out separately from each other. However, the pre-wetting treatment (first pretreatment) and the replacing treatment (second pretreatment) may be carried out simultaneously by immersing a substrate in a deaerated pretreatment solution in the range from a mildly acidic level to a mildly alkaline level, with copper dissolved therein.

In the above embodiment, copper is deposited by way of replacement on a surface of a cobalt film, which doubles as a barrier layer and a seed layer, and via holes are embedded with copper by a plating process. However, the present invention is also applicable to the deposition of copper on a surface of a metal which has a greater ionization tendency than hydrogen according to a plating process. In such a case, copper is deposited by way of replacement on the surface of the metal using a pretreatment solution with copper dissolved therein, and then the surface is plated with copper. Although the cobalt film of this embodiment functions as a barrier layer, a nickel film may be used as a barrier layer. The metal (first metal), which covers the via holes, may no necessarily function as a barrier layer. The present invention is also applicable to a process of forming another barrier layer, depositing a metal (first metal) which has a greater ionization tendency than hydrogen on the barrier layer, and then plating with copper on the deposited metal. The metal to be deposited by way of replacement is not limited to copper, and the metal to be embedded in the via holes is not limited to copper.

Examples of the metal (first metal), which has a greater ionization tendency than hydrogen according to the present invention, include nickel, for example, in addition to cobalt. Examples of the metal (second metal) to be deposited on the surface of the first metal by way of replacement include palladium, gold, and silver in addition to copper. In the above embodiment, a metal is embedded in via holes. However, the principles of the present invention are also applicable to the embedding of a metal in trench interconnects or the plating of through holes.

While the present invention has been described with reference to preferred embodiments, it is understood that the present invention is not limited to the embodiments, but is capable of various modifications within the general inventive concept described herein. 

1. An electroplating method comprising: preparing a substrate having via holes covered with a first metal, which has a greater ionization tendency than hydrogen, in a surface thereof; pretreating the substrate by immersing the substrate in a pretreatment solution in which a second metal that is more noble than the first metal or a salt thereof is dissolved; and then electroplating the surface of the substrate to embed the second metal or a third metal in the via holes.
 2. An electroplating method according to claim 1, wherein the pretreatment solution is in the range from a mildly acidic level to a mildly alkaline level.
 3. An electroplating method according to claim 1, further comprising: pretreating the substrate by immersing the substrate in deaerated water before pretreating the substrate by immersing the substrate in the pretreatment solution in which the second metal or the salt thereof is dissolved.
 4. An electroplating method according to claim 1, wherein the first metal is cobalt.
 5. An electroplating method according to claim 1, wherein the second metal is copper, and the pretreatment solution has a copper concentration ranging from 1 to 70 g/L.
 6. An electroplating method according to claim 1, wherein the second metal is palladium, cold, platinum, or silver, and the third metal is copper.
 7. An electroplating method according to claim 4, wherein the second metal is copper, and the pretreatment solution has a copper concentration ranging from 1 to 70 g/L.
 8. An electroplating method according to claim 4, wherein the second metal is palladium, cold, platinum, or silver, and the third metal is copper. 